1. Field of the Invention
The present invention relates to an apparatus and a method for detecting the position of a laser beam.
2. Discussion of the Prior Art
In systems employing laser beams it is desirable in some applications to be able to change the direction of the beam using one or more mirrors. In some cases it is necessary to be able to measure the position of the beam accurately relative to a datum point on the mirror.
Previously reported research has shown that acoustic waves are produced when a laser beam is incident upon a mirror employed to deflect the beam. These signals have been attributed to thermal absorption in a thin surface of the mirror. Owing to thermal expansion a pressure is exerted in the surrounding area. The thermal energy is deposited so rapidly by the beam that the adjacent material has no time to relax, and hence quench the acoustic wave. The laser beam thus acts as a source of elastic waves on the surface of the mirror.
It has been suggested in the prior art that the acoustic signal produced by a laser beam in a reflector may be used to detect a predetermined characteristic of the beam, particularly the beam power. For example, prior patent specification GB 2158942A describes such an arrangement in which the acoustic signal is proposed to be picked up by an electromechanical transducer mounted on a beam reflector. However, no practical application of this idea has been made hitherto because no practical way has been proposed of putting it into effect.